We have found that a potent tumor promoter, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) can either enhance or inhibit the proliferative response of activated lymphocytes in vitro. We have preliminary evidence that the inhibitory activity is mediated by cells or cellular materials. We propose to determine the cellular mechanisms by which TPA blocks the response to the lectins, phytohemagglutinin (PHA) and concanavalin A (ConA), and to allogeneic cells during a mixed lymphocyte response (MLR). Populations of TPA-treated and untreated bovine lymph node lymphocytes will be isolated either by selective killing or by physical separation. Velocity sedimentation (1 g), column and cell adherence techniques will be used. Cells and their conditioned media will be tested for their effect on DNA synthesis in lectin and MLR cultures. If a class of cells is found to be responsible for the effects of TPA, we will attempt to identify it and to determine if it acts by production of an inhibitory material. The properties of the factor will be compared with some known lymphokines and products of TPA-treated cells. We will attempt to isolate this material. Some other tumor promoters related to TPA will be tested in the lymphocyte system to determine the correlation between tumor promotion in vivo and lymphocyte inhibition in vitro. In addition, compounds which are not promoters but can mimic some of the effects of TPA, and some compounds which have in vivo anti-promoting properties will also be tested.